The present invention relates to a method and an arrangement for reducing the influence of distortion in connection with analogue-digital conversion.
In modern electronic systems for information transmission, the information is often transferred on frequencies different to the original. During signal processing of the transmitted signal, the signal is thus returned to the original frequency. The transmitted signal can, for example, be a carrier wave which is modulated with the information signal.
In connection with signal processing, it is often desirable to detect both the amplitude and phase of the information signal. In such cases, quadrature detection is employed, whereby the signal is mixed down (converted) to the base band in two parallel mixers with 90.degree. phase difference, i.e. to the original frequency of the information signal. Accordingly, during detection two 90.degree. phase-shifted signals, normally termed I (In phase) and Q (Quadrature), are obtained. The signals are converted to digital form in an A/D-converter for each signal and thereafter stored together, whereby both amplitude and phase information is attained.
So that both the amplitude and the phase information is correct, the processing of the I-signal and the Q-signal must occur in identical channels.
However, during the A/D-conversion, distortion/interference arises i.e. in the form of harmonics of the input signal, primarily due to the fact that the conversion is not linear. A mirror (image) tone of the input signal arises even though differences are present in, for example, amplification between the two A/D-converters.
Normally, the mixing down (conversion) to the base band takes place using a local oscillator signal with a constant frequency. The distortion which arises can thus affect the working (utility) signal which coincides with the frequencies of the distortion which arises.
An earlier known arrangement for reducing the influence of distortion is described in Swedish patent application SE9100501-7 which corresponds to U.S. Pat. No. 5,216,425. According to this document, I and Q are phase-modulated before the A/D-conversion and demodulated after the conversion. The phase modulation is altered randomly between each sample which is to be converted. By using this method, the energy content of the distortion products will be spread out over a relatively wide band and it will thus be possible to filter out the information carrying signal, the working signal, with a narrow band filter. Since the energy content of the distortion products is spread out over a wide band, the noise level will be raised. If the distortion products are large, an unacceptably high noise level will thus arise. The flexibility of the arrangement to be used for different forms of distortion/disturbance is thus limited.